The present invention relates to phase measuring methods and apparatuses for optical thin films. The present invention is suitable, for example, for measurements of a phase of an optical thin film applied onto reflection and transmission surfaces of an optical system in an exposure apparatus of a step-and-repeat manner, a step-and-scan manner, etc. for fabricating devices, e.g., semiconductor devices (such as ICs, LSIs, etc.), image pick-up devices (such as CCDs, etc.), and display devices (such as liquid crystal panels, etc.).
Along with the fine device patterns, an exposure wavelength for projecting and exposing a device pattern onto a photosensitive material is becoming shorter and shorter. For example, an exposure wavelength is in transition from KrF (with a wavelength of 248 nm) to ArF (with a wavelength of 193 nm) and F2 laser (with a wavelength of 157 nm), and even EUV light with a wavelength of 13.4 nm has also reduced to practice.
A smaller device pattern is the most important factor that supports the dynamics in the semiconductor industry, and thus the age requiring for the resolution with a critical dimension (“CD”) of 0.25 mm for DRAMs has rapidly changed to that of CDs of 180 nm, 130 nm, and even 100 nm. The lithography using an i-line (with a wavelength of 356 nm) as exposure light has never required the resolution with a CD less than a wavelength.
On the other hand, KrF with a wavelength of 248 nm has been applied to the lithography that requires a CD of 180 nm or even 150 nm. The resolution with a CD less than a wavelength is about to reduce to practice by exploiting achievements in improved resists and the super-resolution techniques, etc. Various super-resolution techniques would possibly realize the pattern resolution with a CD of a ½ wavelength in the line and space.
However, the super-resolution technology is often subject to pattern manufacture restrictions, and the most effective way of improving resolution is, after all, to use a shorter wavelength as exposure light and a higher NA for a projection optical system. This fact greatly motivates for shorter wavelengths of exposure light, and has led to a development of the EUV lithography that uses light with a wavelength of 10–15 nm as exposure light.
An optical system that receives EUV light as exposure light may use only limited materials since no materials transmit the EUV wavelength range. In particular, the EUV region does accept transmission type optical elements, but requires reflection type optical elements with optical constant of nearly 1 and low reflectance on their mirror surface. A characteristic of a reflection enhancement film applied onto a mirror is an important factor to secure expected reflectance. The film material is also subject to many restrictions, and the typical fundamental structure is a film (i.e., a multilayer film) of Mo and Si alternate layer. Other multilayers include, for example, Be—Si and Rh—Si multilayers. The film requires 40 multilayer pairs, each pair having a Mo and Si alternate layer, and provides extremely drastic changes of optical characteristics.
Characteristically, a film for the EUV region requires phase (or phase-distribution) control as well as its reflectance. Whenever EUV light reflects on the film, its phase changes. The phase distribution in the film distorts a wavefront that enters a mirror surface, and an offset in cycle length or a film's quality change on the mirror surface would cause (wavefront) aberration. It is thus desirable to measure the film's phase (or phase distribution) as soon as the film is applied onto the mirror surface. It is particularly desirable to measure position-dependent and angle-dependent phase characteristics of the film.
A correction of aberration in a reflection imaging system particularly requires accurate control over not only precise shaping of a mirror plate for its curvature, aspheric amount, etc., but also the performance of a reflection-enhancement multilayer film. The accurate control over the multilayer film requires a phase of reflected light not to greatly change according to reflected positions on the same mirror surface.
Practically, an angle of light incident onto a mirror is so small that incident light mostly rests within an angular distribution range of several degrees at least and about 20–30 degrees at most. Measurements of a phase distribution with various incident angles onto a film on a mirror surface would be difficult due to arrangement restrictions.
Before the reflection imaging system is assembled, it is necessary to elaborate a mirror shape, and check if reflected phases at respective positions on the multilayer film above the mirror surface are pursuant to the design values.